This invention relates to a controller for regulating the volumetric rate of flow of air through a duct. More particularly, the invention relates to a controller for maintaining a substantially constant volumetric rate of flow of air at a predetermined sensed condition and for resetting the constant volumetric flow rate as a function of at least one sensed condition.
A controller of this general type is disclosed in Maxson et al U.S. Pat. No. 4,264,035. In that controller, a pneumatic actuator resets the flow rate in response to changes in a control pressure signal which varies over a predetermined range of pressures. When the controller is calibrated to cause the air flow rate to vary across a maximum span, substantially the full range of the variable control pressure is available to effect the change. If the controller is adjusted, however, to reduce the air flow span, only a portion of the control pressure range may be utilized. As a result, the system is very sensitive at lower air flow spans and thus the air flow tends to change abruptly from one extreme to the other rather than modulating gradually between the two extremes.